Inkjet recording methods enable simple and inexpensive image formation, and therefore have been used for the formation of various images. One example of inks to be used for the inkjet recording methods is an actinic radiation-curable inkjet ink. As for the actinic radiation-curable inkjet ink, most part of the ink component is cured due to the irradiation with actinic radiation such as ultraviolet rays. Accordingly, discharged ink is immobilized more easily than a solvent-based ink composition, so that an image is less likely to bleed. Therefore, the actinic radiation-curable inkjet ink has an advantage of enabling images to be formed on various recording media.
Image formation by means of inkjet recording method is typically performed by discharging an ink from a recording head of an inkjet recording apparatus. However, when the ink contains air bubbles in discharging the ink from the recording head, discharge deficiency may occur. Here, the above-mentioned actinic radiation-curable inkjet ink, in particular, an actinic radiation-curable inkjet ink containing a wax has higher viscosity than a solvent-based ink, and thus air bubbles mixed thereinto during ink production or storing are not easily degassed. This tends to cause the deficiency of ink discharge from an inkjet recording apparatus (e.g., Patent Literature (hereinafter, referred to as “PTL”) 1).
Examples of proposed method of removing air bubbles contained in an ink include a method of degassing an ink by allowing it to flow through a hollow fiber degassing module before filling a cartridge with the ink (e.g., PTL 2); and a method of degassing an ink immediately before being discharged with a degassing filter provided inside an inkjet recording apparatus (e.g., PTL 3). Further, a method of degassing an ink by spraying the ink into a depressurized space provided inside an inkjet recording apparatus (e.g., PTL 4) is also proposed. Furthermore, a method of degassing an ink by allowing the ink to flow little by little for a long period of time inside a depressurized space provided in an inkjet recording apparatus (e.g., PTL 5) is also proposed.